In recent years, image display apparatuses, such as liquid crystal projectors and digital light processing (DLP) projectors, are increasingly used for systems to project broadcast and played-back images or to make presentations with the use of personal computers.
Such image display apparatuses use, as a light source, a short-arc high-pressure mercury lamp (for example, Japanese patent application publication No. H02-148561) which is substantially a point light source.
This high-pressure mercury lamp includes a light emission part having a pair of electrodes therein, and sealed parts which extend from different ends of the light emission part. In each of the sealed parts, a metal foil connected to a corresponding one of the electrodes is sealed airtight (sealed foil structure). In the light emission part, a predetermined amount of mercury, which is a light emitting substance, and a predetermined amount of a halogen to cause a halogen cycle are at least enclosed.
To improve the lifetime and illuminance of the high-pressure mercury lamp, it has recently been attempted to enclose the mercury and halogen at a higher pressure.
However, this attempt poses the following problem.
In the high-pressure mercury lamp, there is a slight gap between a portion of each electrode rod which is placed in a corresponding one of the sealed parts and quartz glass forming the sealed part. Here, a degree of airtightness of the sealed part is dependent on a degree of adhesiveness between the metal foil and the quartz glass forming the sealed part. If the pressure in the light emission part is high, the metal foil and the quartz glass increasingly separate from each other as time elapses after the high-pressure mercury lamp is lit. This causes the enclosed contents in the light emission part to leak out.
To prevent such leakage, Japanese patent application publication No. 2002-93361, for example, suggests the use of VyCor glass (product of Corning, Inc., Japanese registered trademark No. 1657152) including silica (SiO2) principally, and additionally contains aluminum oxide (Al2O3), boron oxide (B2O3), sodium oxide (Na2O), and the like. In detail, VyCor glass is provided between quartz glass forming each sealed part and a portion of an electrode rod forming an electrode, which is placed in the sealed part. The electrode rod is tightly adhered to the VyCor glass by the sealing process. This prevents a metal foil and the quartz glass from separating from each other, thereby avoiding leakage of enclosed contents in a light emission part.
However, this high-pressure mercury lamp using VyCor glass to allow high-pressure enclosing has the following drawback. When the high-pressure mercury lamp is lit in such a state that a lengthwise axis of a translucent vessel including the light emission part and the sealed parts extending from different ends of the light emission part is substantially orthogonal to the vertical direction, significant devitrification occurs in quartz glass forming an upper portion of an internal surface of the light emission part. As a result, the luminous flux is reduced, and the high-pressure mercury lamp is swollen due to the devitrification, to be deformed or broken. In addition, the internal surface of the light emission part significantly blackens at an early stage of the lighting.
Such a high-pressure mercury lamp conventionally uses methylene bromide (CH2Br2) for a halogen. Alternatively, Japanese patent application publication No. 2001-338579, for example, suggests the use of mercury bromide (HgBr2) in order to prevent interfusion of impurities such as carbon and hydrogen into the light emission part.
However, it has been confirmed that a high-pressure mercury lamp in which mercury and mercury bromide as a halogen are enclosed also has the above problems. Specifically speaking, quartz glass forming an upper portion of an internal surface of a light emission part significantly devitrifies. As a result, the luminous flux is reduced, and the high-pressure mercury lamp is swollen due to the devitrification, to be deformed and broken. In addition, the internal surface of the light emission part significantly blackens at an early stage of lighting.
Japanese patent application publication No. 2001-266797 discloses a construction that a conductive heater is wound around each sealed part in a high-pressure mercury lamp of a direct-current (DC) powered type. The conductive heaters are supplied with power before the high-pressure mercury lamp is lit, so as to heat the sealed parts. This has a purpose of shortening a time period required for the lamp to start emitting light after power supply starts and preventing glow discharge which occurs at the start of lamp operation. The disclosure includes an embodiment in which an external lead wire provided in a sealed part on a cathode side is electrically connected to a conductive heater provided for the sealed part at the cathode side. The conductive heater, which is wound around an external surface of the sealed part at the cathode side, has a lower potential than an electrode rod which is placed in the sealed part at the cathode side, because of a voltage drop due to electric currents flowing in the electrode rod and metal foil which are placed in the sealed part at the cathode side. Therefore, an electric field is created between the electrode rod and the conductive heater. As a result, alkaline components in the sealed part are extracted toward the external surface of the sealed part at the cathode side, around which the conductive heater is wound.
It is generally understood that, when the alkaline components exist at an interface between the metal foil made of molybdenum and quartz glass forming the sealed part, connection between the molybdenum and the quartz glass is cut. This lowers connection strength between the metal foil and the quartz glass, thereby lowering the lamp's pressure resistance. According to the above-described construction, the alkaline components are extracted toward the external surface of the sealed part at the cathode side, which prevents a drop in the lamp's pressure resistance.
However, when the above-described construction is applied to a high-pressure mercury lamp with a high output, especially, a rated power of 200 W or more, the following problem emerges. When the high-pressure mercury lamp is lit in such a state that a lengthwise axis of a translucent vessel is substantially orthogonal to the vertical direction, quartz glass forming an upper portion of an internal surface of a light emission part significantly devitrifies. Therefore, the luminous flux is reduced, and the lamp is swollen due to the devitrification, to be deformed or broken. As a result, the high-pressure mercury lamp can not achieve a rated lifetime (2,000 hours). Even when the above-described construction is applied to a high-pressure mercury lamp with a rated power of less than 200 W, the same problem emerges. When the high-pressure mercury lamp is lit for more than 2,000 hours, devitrification becomes noticeable. Therefore, this high-pressure mercury lamp can not attain a long rated lifetime of 10,000 hours or more, and therefore can not meet the demand in the related market.
In particular, irrespective of output power, the above-described problems are found in a high-pressure mercury lamp in which VyCor glass is provided, so as to be tightly adhered to the electrode rods of the electrodes, between quartz glass forming sealed parts and portions of the electrode rods which are placed in the sealed parts, and a high-pressure mercury lamp using electrodes which contain alkali metals of 12 ppm or more as impurities. Specifically speaking, when these lamps are lit in such a state that a lengthwise axis of a translucent vessel is substantially orthogonal to the vertical direction, an upper portion of an internal surface of the translucent vessel significantly devitrifies. In addition, the internal surface of the translucent vessel blackens.
The above-mentioned problems are common to any high-pressure discharge lamps including sealed parts, and are not particular to high-pressure mercury lamps.
In light of these problems, an objective of the present invention is to provide a manufacturing method of a high-pressure discharge lamp which causes less devitrification in a light emission part of a translucent vessel and prevent an internal surface of the light emission part from blackening, even when the high-pressure discharge lamp is configured to have a high output and a high internal pressure. The present invention also aims to provide such a high-pressure discharge lamp, and a lamp unit and an image display apparatus using the high-pressure discharge lamp.